Comments

Perhaps I missed it, but I don’t understand how a reduction in AMOC would lead to the global cooling (however slight, or at least slow down of the warming) associated with YD. I can understand how it might cool the north Atlantic, but not how it would have this larger global effect.

On the possible causes of the slowdown itself–given that something similar has happened at the end of previous glacial periods, isn’t it likely that there is something about the patterns of melt that is associated?

Perhaps melt tends to happen on the surface of the ice sheet creating huge fresh water seas that then dump into the ocean at a great enough rate to slow or stop the AMOC. With the cooling that results, the process starts over until there is not enough glacier left to cause the slowdown.

(OT, a bit. How much fresh water is necessary to stop the AMOC? Could such a figure be accurately calculated. It seems to me I have heard that it takes one Sverdrup. It this remotely accurate?)

Hi Chris, you said that “[u]nlike changes in global temperature (such as modern day global warming) which can be understood as a result of perturbations to the planetary energy balance, the millennial-scale climate changes during the last glaciation are viewed primarily from the lens of internal dynamics, including ice retreat and re-organizations of ocean circulation.”

It is not clear to me, why? It has the appearance of an arbitrary choice. It looks like, “we prefer to explain millenial-scale changes by appealing to internal dynamics and we meanwhile prefer to explain the present global warming by appealing to perturbations of the energy balance.”

Now the IPCC report says, “[i]t is ‘very likely’ that average Northern Hemisphere temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years. It is also ‘likely’ that this 50-year period was the warmest Northern Hemisphere period in the last 1.3 kyr, and that this warmth was more widespread than during any other 50-year period in the last 1.3 kyr.”

Now after decoding the meanings of ‘likely’ and ‘very likely’ I am left with the conclusion that actually we just aren’t certain at all when it comes to global average temperatures and the last thousand years.

So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

So what can we possibly know of ‘global average temperatures’ from as far back as the Younger Dryas? Surely, the further back we go the less certain our knowledge of the global average ‘anything’ becomes.

Where am I wrong here?

Alex, let me try.

Yes, if you want to compute a well defined global average temperature to a precision of tenths of a degree, then having a good, global geographic distribution of sensitive temperature proxies is critical. And yes, this will get you into trouble already for the first millennium AD — for the current state of the proxy data. The IPCC statement you refer to tells more about the current state of the evidence (and remember, even “likely” is a lot better than not knowing at all — a military commander would kill for such knowledge!) than about reality back then… don’t place your hopes on the “God of the Gaps”, it won’t last :-)

About YD, I see that in the article initially only temperature data (isotope proxy) from two continental ice sheet sites are used; global averaging isn’t even attempted (and would be of limited usefulness due to the asymmetry of the pattern).

Then, in the glacial maximum / interglacial temperature contrast and transition study, again proxy data from many locations are used — but here we talk about temperature differences of several degrees; easier, even though much further back in time.

My guess at #1 (wili) is that northern cooling leads to extended ice cover, lower albedo, and hence lower total insolation. I’d also guess that the cooling affects both hemispheres, so it’s a double whammy. Both could in turn reduce CO2 levels (perhaps by covering over extensive plant growth without giving it a chance to decay and enter the atmosphere), or by affecting ocean CO2 levels by significantly cooling the oceans nearer the poles.

Question: what would re-accelerate the AMOC? How did the YD end? And did an acceleration of the AMOC end the YD, or vice versa? I’d expect that that would also be a clue (or at least supporting evidence) as to how the YD started.

Lastly, has anyone quantified the AMOC and its relationship to temperature (such as by setting the “fast” and “slow” AMOC rates in Sverdrups, and relating that to total change in annual hemispheric heat transport, e.g. W/m2/year)?

Denton, one of the first reference et al.s, recently published “The Last Glacial Termination” with a different set of et al.s. Denton seems to have been chasing termination triggers and couplings for quite some time now.

A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres…”

Heinrich I and Younger Dryas stadials are connected to melt-water/iceberg outbursts and eventually to ocean released CO2. Interesting read…

Okay, quick backtrack. I do need to remember to read every article 3 times before posting (and I’ve only read it twice now, so I’m already breaking that rule, again…)…

My stupid comment #9 ignored Chris’s explicit statement in the original post that the North became colder and drier, while the South became warmer and wetter, and global temperatures only varied by ~0.6˚C, so my revised comment to wili (#1) becomes… the article didn’t say global cooling, it said regional cooling.

Alex @3.
If we think in terms of perturbations on a sphere. The simplest perturbation is L0, which is a uniform global warming/cooling. The next axisymmetric perturbation would be L1, cooling at one pole, and heating at the other. For a global driver, such as a change in solar intensity or a change in greenhouse gas concentrations the driver is primarily L0. If AMOC transfers heat from one hemisphere to another, and the strength of this transfer changes the driver would be primarily L1. The largest response mode is likely to be similar to that of the driver, so we would expect changes of global drivers to affect mainly L0, and changes in internal heat transfers to affect mainly L1. The reported changes during YD are primarly L1 (amplitude 2.5C), with some spillover to L0 (amplitude .6C). Primarily because the latitudinal distribution of land surface is very assymmetric (more land in the northern hemisphere), it is not unexpected that global temperatures would be affected as well, as northern hemisphere albedo is more temperature dependent than southern hemisphere albedo.

I thought it was an event due to the ice plugged lake abruptly collapsing and releasing it contents into the north atlantic via a natural warming which causes an abruot reversal in the due to the weakening of the THC.

However I am presuming that the actual idea of the trigger of a YD eventis presently not understood and not likely a comet so some other mechanism?

Just as the ice under an ice skater’s blade melts under the pressure, ice under a head of ~ 6 meters melts under the pressure of the water above it. Thus, an ice dam cannot form a lake of (liquid) water deeper than ~ 20 feet. If we are looking for a lot of fresh water retained by an ice dam, the lake would have to be broad rather than deep. The unpleasant possibility is that a shallow lake fell through the ice sheet and rapidly mobilized a relatively large volume of ice.

Could the wave terraces at Lake Missoula have been formed by a shallow lake sitting on a thick wedge of ice?

A consistent indirect effect of the initial warming at the end of recent ice ages appears to be a meltwater-mediated northern hemisphere hiatus in the warming.

That much ice, melting at that rate, and running off North America, appears to have the consistent and predictable effect of slowing the AMOC (which I would have naively termed the gulf stream), and thereby generating a pause in the warming for the northern hemisphere.

And if you just look at the big sonofagun of a blue dot above, and read the text, we’re talking about drainage of the Canadian ice shield, via the Great Lakes and the St. Lawrence river.

So, no dramatic event is needed, it’s just a routine consequence of storing then releasing that much water off the land surface of Canada.

Knowing none of the detail, that all seems completely reasonable to me.

Now I’m back to comment #1. Can you use this new information to help triangulate our current situation. We don’t have a Canadian ice shield now. All we have is Greenland. Is there any reasonable back-of-the-envelope calculation that rules out such an effect from a much more rapid melting of a much smaller amount of ice?

So now I’m trying to trade off area versus time, from the charts above. (Mercator projections are unhelpful). And my response is, basically, hmmm.

Unless I’m reading this wrong, the land area of Canada is 4.8x the land are of Greenland.

So, if Greenland melts 4.8x as fast as the Canadian ice shield did at the end of the last ice age, then … we ought to get about the same effect, … eh?

Not knowing the depth of the ice, and so on and so on, I’m brought back to what James Hansen has said about sea level change. Yeah, sure, the average change is slow. But, historically, you’ve seen periods of rapid change, and, hey, that just might happen to your grandkids. So don’t take the average change as indicative of your (immediate progeny’s) risk from this.

Secondarily, maybe the east coast of NA will be spared the worst of it, for a century or two, as the Greenland ice sheet melts?

Christopher Monckton and other deniers get far more press coverage than they deserve. Journalistic false balance has caused the public to be confused on climate change – the greatest threat to humanity this century. Worse, these deniers have used mainstream media to attack climate science and the scientists who pursue the truth. Let us now turn the tables.

Monckton has been exposed by Dr. John Abraham and instead of hiding his tail and whimpering away, Monckton has gone on the offensive by attacking Dr. Abraham and asking his followers to essentially “email bomb” Dr. Abraham’s university president. We need to alert the media to this story.

I have assembled a list of 57 media contacts in the hopes that my readers will follow my lead and send letters asking for an investigation of Monckton and his attack on Abraham. I have placed mailto links that will make it easy to send letters to several contacts at once with a single click.

In the thread comments, please suggest other contacts in the US and from abroad. This blog thread can then be used in the future to alert the media to denialist activity.

Great idea! (I’m presuming you’re managing this in such a way as not to “bombard” journalists.)

On a parallel thread here, Geoff Wexler makes some good points about “defensive writing,” but we can’t always be on the defensive. You’re playing some offense here, by being proactive in disseminating accurate information about denialist tactics.

It will of course be called “politics”–which it is, politics being how human affairs are managed. But it will be written/said sneeringly! Par for the course.

#4 Brian Lux: Definitely a hard blue, if not black one…
introduction to black, perhaps – rewiew on recent very technical papers on paleoclimate, it looks like. I’ve never actually made clear to myself what the names of different periods refer to but this post (I think) makes it easier to go look for possible explanations and actual records.

The articles on the Megafauna extinction was interesting but received far too much attention in secondary sources with respect to causing the Younger Dryas, or trying to extend the “Anthropocene” for that matter. Even in the primary article the authors seemed quite unclear about distinguishing between global vs. Greenland impacts, and the methane changes at the YD are too small to have a significant global effect.

Can I just throw it out there that although there is a lack of IRD spikes during the YD, I don’t think calving instabilites can be ruled out. Just look at how sensitive marine terminating glaciers are: the PIG is loose, SE Greenland recently doubled its speed and Alaskan tidewater glaciers collapsed out of the little ice age. Drainage re-routing and megafloods are certainly important, but Terminations occur by Milankovich driven calving episodes.

Bob at 12, thanks for the thoughtful responses. Note that, parenthetically at least, I noted that the warming was slight and that even a slow down or temporary pause in the global temperature increase needs to be explained.

It looks as though the most we can ‘hope’ for, as far as a similar pause to our current warming goes, is a temporary cooling, or a slowing of the warming, mostly in the north, but at the cost of greatly and quickly increased sea levels.

But even such a slow down may help slow or postone the crucial methane feed backs from tundra and Arctic continental shelves.

At this point, I find myself grasping at any possible unknown (or barely known) unkowns that might step in at the last minute and mitigate what is looking more and more like a biblical or worse catastrophe facing the planet. Responses of most individuals, institutions and nations to the crisis seem tepid at best given the gravity of the situation.

And I join in thanking Mandia for his attempt at organizing a response to the disinformation and smear campaign.

Back-of-the-envelope calculations are a good way to ensure that claims like this are reasonable or not.

Consider an ice sheet 3000m thick, and for the sake of simplifying calculations, has a density of 1000 Kg/m^3. The pressure on the surface below the ice sheet due to the ice sheet’s mass is (density)*(gravity)*(height) = (1000 Kg/m^3) * (9.8 m/s^2) * (3000m) = 2.94 x 10^7 Pa, or 29.4 MPa.

Chris, very exciting article. It leads to even more exciting questions about the mechanism for the termination phase. It will be fun to see what is explored from these new directions, and of course new information and articles yet to come.

I have updated the Lindzen page with some dialogue from Ben Santer and Mike Mann from the Marketplace forum held on June 9th.

I have transcribed the Q&A from the Marketplace web site video of the session. My main goal with the question was to get something in the public that addressed the ‘Iris Hypothesis’. Both Mike & Ben contributed to the answer.

18:
I think what was meant was that at a differential pressure (deviatoric stress) of approximately one bar (about 10meters of water) that ice can be considered to flow plastically. If you place a lake of water on a ice sheet, after its depth reaches roughly 100M it has enough pressure to push its way through. This places a constraint on icedam height.

#23 There is no firm evidence about humans “causing” megafauna extinction anywhere in the world. It is much more likely that climate change caused the extinctions. In the great scheme of things, the “reduction in methane from megafauna” is unlikely to be more than a very minor feedback, particularly since the megafauna absence would be balanced by increases in numbers of other herbivore species.

There was a similar kind of hypothesis in Australia, but in that case it was that the lack of megafauna caused more grass to not be eaten and therefore more human fires were needed (see http://davidhortonsblog.com/history/). Absolute nonsense, again, not least, because the absence of megafauna would be compensated for by increasing kangaroo numbers.

These kinds of theories present a curiously static view of the way ecosystems work.

“the methane changes at the YD are too small to have a significant global effect.” – Chris Colose

Hmmm… that’s not at all what I would have said. In fact, the graphs of methane and CO2 at the Younger Dryas should have been incorporated in this discussion. Furthermore, those increase took place over a 1,000 year period – while modern climate models barely project 100 years into the future, a result of the lack of biogeochemical modeling efforts.

ScienceDaily (Apr. 24, 2009) — An expansion of wetlands and not a large-scale melting of frozen methane deposits is the likely cause of a spike in atmospheric methane gas that took place some 11,600 years ago, according to an international research team led by Scripps Institution of Oceanography at UC San Diego.

From previous work:

During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments.

Clearly, methane was a key player in the warming associated with the Younger Dryas, regardless of the claims made in this post. See the following:

[Response: I’m with Chris on this one. The ice core evidence is clear that methane was strongly affected by the YD (and indeed by other D-O events), but the radiative forcing involved in those changes are too small to have any significant effect on the global temperatures. Look at the graph, the change in CH4 is around 200 ppbv (about a fifth of the anthropogenic change since 1750), and has a forcing of ~0.14 W/m2, equivalent to a 0.1 deg C temperature change at equilibrium – far smaller than the actual temperature changes. – gavin]

One more comment on aaron (16) – the idea that the pressure of an ice skaters blade causes the ice to melt and thus makes ice slippery is not correct. The pressure of a skaters blade only reduces the melting point by a few degrees, but ice is slippery down to about -40 celsius. Exactly why ice is slippery is still a research question, it seems that there is a thin layer of liquid water on the surface of ice even at temperatures well below zero.
see, e.g. http://people.virginia.edu/~lz2n/mse305/ice-skating-PhysicsToday05.pdf

Interesting mention of Faraday, working on the problem of regelation. John Tyndall, the first to measure the greenhouse properties of various gases, including CO2, was interested in regelation, believing it to be an important mechanism in glacial movement.

I’m uncomfortable with the interpretation of EOF2 as a “real” mode of climate variability. Since EOF1 represents much of the variance, and is in the same phase globally, and since EOF2 must be orthogonal to EOF1, and there is more variance at the poles than near the equator, EOF2 will be a hemispheric pattern with the axis of variation forced to line up with the poles. This will be robust to statistical jacknife procedures, but it has limited physical reality. In other words, the EOF process does not confirm the physical reality of the bipolar see-saw, but is forced by the concentration of variance at polar latitudes to look like a see-saw, regardless of what is really happening in the climate system in the way of “non see-saw” behavior.
To quote from the abstract “Empirical Orthogonal Functions: The Medium is the Message” by A. Monahan, et. al., Jour. of Climate, 15 Dec. 2009, 6501-14 : “…in general individual EOF modes (i) will not correspond to individual dynamical modes, (ii) will not correspond to individual kinematic degrees of freedom, (iii) will not be statistically independent of other EOF modes, and (iv) will be strongly influenced by the nonlocal requirement that modes maximize variance over the entire domain.”
I am not saying that the see-saw does not exist. I am saying that the act of running EOFs on the data and treating EOF2 as “real” will tend to disguise other aspects of the climate dynamics that do not resemble EOF2.
That doesn’t strike me as helpful in trying to understand how the climate system was operating during this period.

> . If you place a lake of water on a ice sheet, after its depth
> reaches roughly 100M it has enough pressure to push its way through.
Reasonable
> This places a constraint on icedam height.

Don’t forget that the weight of 100 meters of ice creates about 90% of the pressure of the water, and that the plastic flow of the ice will create a side pressure opposing the water pressure at the bottom of the ice dam. A 1 km thick ice sheet could retain about a 900 meter deep lake, at least until it flowed downhill enough to fail

Does anyone know the truth on this? WOWT and a dozen other anti-science blogs are reporting that the U.S. Department of Energy is suspending their funding of CRU, which would be both a crime against humanity, silliness (considering all of the official reports clearing them), and lastly a black mark on the administration (did G.W. get back in office?).

The only “credible” source (say that with muffled laughter) that they link to is a story by good old stalwart Jonathan Leake at the Sunday Times, but the story is behind a pay wall.

Does anyone know the facts/substance behind the story? Is this as simple as him twisting a standard procedural step, or pulling out old news (like a suspension while things were under investigation) and acting as if it’s new, or just plain making it up?

RE #37 – “Does anyone know the truth on this? WOWT and a dozen other anti-science blogs are reporting that the U.S. Department of Energy is suspending their funding of CRU”.

The most effective propoganda uses the same phycological tool as magicians do, they lead their audience into accepting unstated assumptions. A quick google confirms the CRU is funded by the Natural Environment Research Council, which as one would expect is a British government agency doing it’s job funding a British university.

Whatever the final explanation turns out to be, whether hydrogeological or cosmological, I’m still fairly well convinced the locus tis thing is just south of Lake Nipigon. The proxy work that has purportedly refuted the comet impact hypothesis has been sparse and located far from the proposed impact site, and only refutes the widespread comet shower with continental wildfires scenario, and not anything more localized and ablative. Continental geography is set up such that if this deglaciation scenario has repeated itself through multiple glaciations, then the locus would be roughly right where we find this highly indicative set of unusual geological features as well. So it truly is an unresolved question, and clearly further work is coming.

In reference to Thomas (#39) and the statements “Evidence of a specific flood water pathway at the right
time has proven to be elusive. No clear evidence exists for a flood event into the Atlantic. . .
Many erractics (large boulders different from surrounding rock and landscape) exist in present day water courses into Lake Nipigon. The timing of these is 11,000 years before present is very close to the Younger Dryas.

“Although EU funding is very important, we also endeavour to maintain the diverse pattern of funding reflected by the research described in this “history of CRU” and in the list of Acknowledgements ….”

at the bottom of the page just after the words

“This list is not fully exhaustive, but we would like to acknowledge the support of the following funders (in alphabetical order) ….”

Thank you so much for your post on the Younger Dryas. I picked up some time ago from various reading that a sudden emptying of Lake Agassiz was likely to be involved in the reduction of the AMOC, and that lodged in my mind, and I repeated it to others on several occasions. However, the recent research you summarise indicates otherwise, and so clearly I will need to adjust my internal narrative and external communication about this.

Funding of CRU/UEA: If you look at their own site,http://www.cru.uea.ac.uk/cru/about/history/ ,
the list of funders begins with “British Council, British Petroleum…”, goes on to “United Nations Environment Plan (UNEP), United States Department of Energy, United States Environmental Protection Agency…”, and ends with “and the World Wildlife Fund for Nature (WWF).”

Since we are looking at historical climate change here, how should we view Prof. Robert B. Laughlin’s “don’t concern yourself about climate change, it’s not something we can control” article titled “What the Earth Knows”, published recently in The American Scholar? Agree or not, it’s an interesting read. BTW, he is a Stanford University physicist and a co-recipient of the Nobel Prize for physics 1998.

[Response: Hmm… on geologic time scales nothing that we care about will matter, and the planet will still be here. But I have a fondness for the here and now, and so whether the CO2 has been absorbed by the deep ocean in 20,000 years time is less important than what it does to the temperatures and the ice sheets in the meantime. – gavin]

Don’t forget that the weight of 100 meters of ice creates about 90% of the pressure of the water, and that the plastic flow of the ice will create a side pressure opposing the water pressure at the bottom of the ice dam. A 1 km thick ice sheet could retain about a 900 meter deep lake, at least until it flowed downhill enough to fail

Yes, in theory. Two problems:
1. It presupposes that the water level stays 10% below the ice level. There would be special geometries where that would be so, but in general one expects the thing to fill to the lowest level on the crest of the ice dam. At that point basal water pressure exceeds the basal ice pressure, and crack propagation should commence. It may be slow, because ice is stiff and can transfer load laterally. Maybe on-going ice flow could resist it somehow.
2. It presupposes an excellent basal seal. As one who has designed dams to resist high heads, I know how tough that is to achieve. At multi-hundred metre heads the slightest leak can grow rapidly by (physical) internal erosion. With ice, add melting due to latent heat transfer and from flow friction. One can imagine a stable leak, but it will be a long and slow one. Surely the physics of that is done and published long ago; maybe even ground-truthed, given recent intense interest?

Just noting that I think some of the Greenland dO18 isotope data has been miscalibrated to temperature. It changes something equivalent to Antarctica in the ice ages (a little more in Greenland during the YD) so the temperature change should be roughly equivalent as well given the similarity in latitude and conditions.

The temperature change should be more like -8.5C in the ice ages and -7.0C or so in the Older and Younger Dryas cool-down. One chart above has Greenland at -18C in the ice ages which is twice too much given the high latitude. This error has appeared in many papers and should be corrected.

[Response: Actually, no. The standard paleo-thermometer calibration was shown to be wrong for glacial-interglacial temperature changes via the borehole temperature reconstructions, and for the more rapid changes via the nitrogen isotope thermal fractionation results pioneered by Sevringhaus and Brook. The explanation is likely to be the shift in seasonality in colder periods (so that most of the snow arrives in summer, rather then all year round as at present). Werner et al (2001) had a very nice model study that provides a convincing demonstration of the effect. – gavin]

Proglacial Lake Agassiz stopped draining to the Mississippi about at the onset of YD. There is no evidence of drainage to the east at that time and as Chris Colose mentioned, now evidence of drainage to the Arctic Ocean. This would force more sea ice out Fram Strait, hleping to hamper deepwater formation.

But even more fresh water was stored in proglacial lakes furhter east. I found (and linked in a comment on the Clovis Comet thread here on RC, near the end) a paper demonstrating massive flooding down the Mohawk/Hudson where the fresh water would then effect the Labrador deepwater formation. The occurred at about the same time as the onset of YD.

But more. The Baltic Ice Lake drained, with the fresh water going to the Nordic Sea, again at about the same time as the onset of YD, possibly causing a salinity crisis there. The amount of water from this first drainage is poorly characterized sine a subsequent flooding removed evidence at the outflow.

Now all three of these locations were set to flood at any time and it might be simply coincidence that all went nearly simultaneously. YD-III looks enough milder to me to suggest that the proglacial ice lakes at that termination were less simultaneous. Which, combined with other mysteries, still leaves open the Clovis Comet possiblity, although that might have been a meteor shower; there is some suggestive evidence buried under the east end of Lake Ontario.

Brian @36, my estimation of 100M of head needed already accounted for the factor of ten due to ice being approx 90% as dense as water. Roughly speaking 10M of water is one atmosphere of pressure.
Also if the water overflows an ice dam, might thermal plus erosive effects enlarge the outlet? We do have modern smaller scale analogies, I believe Lake George in Alaska is dammed every year and breaks out every summer.

“Also if the water overflows an ice dam, might thermal plus erosive effects enlarge the outlet?” Sure.
Its clear from the geologic record of the Missoulan flood left in the Washington scablands that glacial dams can hold back very large lakes and when they fail they do so catastrophically. The way I envision it occurring is that as the ice age ends, large glacial/ice cap structures that have accumulated over thousands of years to great thickness block drainages. the extended preceding cold and thickness of the ice insure that the base is frozen to the underlying geology, and plastic flow has filled all the contours so it’s sealed.

As the climate warms, water begins to accumulate behind the dam. When the meltwater reaches significant depth(i.e., >900 meters for a 1km high dam), the hydrostatic differential starts to deform and lift the ice dam, and cause failures along planes of weakness – the ice will have been accumulating as snow but sliding and deforming downhill, flowing down the drainage – even where the plastic flow has closed any gaps, there will be weaker areas, and these will fail first. I think that because of geothermal heat and possibly frictional heating, combined with the fact that the top of a thick glacier will be cooler because of the adiabatic lapse rate, the bottom will be warmer and weaker, and will be where the failure will first occur. The Greenland Ice sheet is warmer at the bottom than the top[1], and lakes have been observed beneath Antarctica[2].

The lifting and lubrication of the glacier bottom will cause faster flow, and more stress and failure of the moving ice. With a large volume of water available, the glacial dam could rapidly transform into a megaslushy and then a flood.

The relatively low relief [3] that separates south flow to the Mississippi and north flow to the Red River and the Mackenzie River would have limited the depth of a lake forming at the southern edge of the melting ice sheet at the end of the last ice age. I surmise that an ice dam failure that would release this into the Arctic would have been caused by the thickness of the glacial cap decreasing rather than the depth of the lake increasing. I wonder what sea level was at the beginning of the Younger Dryas – low enough to close the Bering Straight? – and also how thick the Arctic sea ice was. I suppose it’s possible that the fresh water flowed over the surface of the ice and into the Fram Straight, with the thermal energy contained in the water melting snow on the surface and the decrease in albedo caused by wetting the surface increasing the total flow of freshwater into the North Atlantic.

Thanks Brian. Your statement about glacial ice being warmer at depth is true near the center of an icecap. Nearer the edges, in an abalation zone ice flow is from up glacier, where the climate is colder than near its edges -especially its equator ward edges. Because ice is nearly incompressible, the adiabatic heating of ice under compression is pretty low. So in these zones, I think the colder ice is advected at depth, and the near surface is relatively warm, reflecting the warmer climate at the ablation zone. The same ice sheet can have both positive and negative thermal gradients with depth. In any case there is no disagreement about the potential for catstrophic release of dammed up water.

Re 18: Hank,
Certainly, ice sheets can support a lot of ice and be very thick. Ice under a lot of ice tends to be cold and strong. However, ice in contact with water is warm and weak. Ice does not do such as good job of supporting liquid water as the moulins on Greenland demonstrate.

Re 27: Nick, If ice is in contact with liquid water, then the ice is at its melting point. Any depression of its melting point means that it – melts (if it has a source for the heat of fusion.)

Re 35: Hank, Physics and observations in Greenland and France suggest that 6 meters is greatest head of liquid water that ice can support. Look around Greenland. These days Greenland has lots of pools of melt water, some kilometers across. How deep are they? None of the pools of melt water on the surface of the ice are deeper than 6 meters. What happens when they become deeper than 6 meters? They fall through the ice – hundreds or thousands of meters of ice. I brought this up to the RC group a few years and go and they agreed that some physicist had made such theoretical calculations, but said nobody had actually observed the phenomenon. Well, now we have multiple good observations of big (liquid) lakes falling through thick ice.

Re 36/56: Brian, A 1 km thick ice sheet will dam and retain a lake of water that is just over 6 meters deep. Then, the water will punch a hole (moulin) through the ice dam. (Just as ponds of melt water punch moulins through the Greenland Ice Sheet.) We really have to change our thinking about Lake Missoula, and etc. Those floods had to start as mostly ice slurries, mobilized by small amounts of water from shallow, super glacial lakes, which fell through thick ice to form high energy streams of water. Then, the ice melted as it moved downhill. This has unpleasant implications as we consider failure modes for extant ice sheets.

Just perusing the uni-bremen site re: arctic ice sheet. Looks like the N.E passage will be open shortly with the N.W maybe at the most a month away again. You could probably quite safely steer an ice breaker though the icy mush of the N.E passage right now.
By the looks of it the summer of 2010 still has the potential to set a new record in extent of ice melt.

It appears that Lake Agassiz emptied to the north, its path there opened by a larger piece of the comet impact.

If you are ever able to look at the temperatures of the Pacific Current at the west coast of North America, the mechanism behind the data will probably become clear to you. Cooler water led to less snow falling in North America, and more sunlight absorbed.

Yes, the front page shows 6.18 and the one of about 2 weeks ago increasing the anomaly from about -10,700 km cubic to about -11,250 never appeared. So here we are 1 month later and still/back on about 10,700 km cubic missing. JAXA is speeding up again too dropping away from the 2009+2006 track.

For the interested, Walt Meier of NSIDC had a guest post a few weeks ago up over on WUWTBOH was comparing his past Sea Ice Volume work with PIOMAS. Interesting, my interpretation being that he gave the thumbs up to the PIOMAS effort.

Thomas Lee Elifritz (54) — I’ve read the attempts to develop evidence for Lake Agassiz outflow to the east at the onset on YD; no evidence found. Several of the papers are linked on the Clovic Comet thread here on RC. A summary is available in Wally Broecker’s “The Great Ocean Conveyor” (2010).

Brian Dodge (56) — Bering Strait was effectively closed at the onset of YD. Not that it mattered much.

It appears that Lake Agassiz emptied to the north, its path there opened by a larger piece of the comet impact.

Ed, the existence of the comet impact is by no means assured. The continent and pole was covered by large ice sheets, periodically waxing and waning for two million years or so. The continental geography hasn’t changed all that much in that time besides the subsiding and rebounding of the lithosphere :

which in the area of interest could have been as great as several hundred meters. Since interglacials are defined by the more or less disappearance of the ice sheet on the continent, more or less all of the ice makes it to some ocean somewhere as fresh water, over a period of time. The question is the amount of fresh water runoff forcing, and over what time period the forcing operates, in order to achieve the observed climatic trigger event. The invocation of cosmological catastrophism may or may not be required, depending on the details of the models describing the effect, and the cometary hypothesis would be much better accepted if there was some definitive evidence for its occurrence, and that remains yet to be demonstrated. So far the only thing in its favor is the detection of nanodiamonds downwind (Greenland) from a presumably more localized impact near somewhere near Lake Nipigon, and the veracity of those claims have yet to be demonstrated. There does appear to be a small blip in the atmospheric nitrogen at the time, but again the resolution of the ice proxies prohibits any definitive statement one way or the other, just like the lake bed and Channel Island proxies do not definitively exclude a more ice and volatile rich impacts at a localized site, contrary to the claims of the authors and the media. The availability of highly resolved ice and sediment proxies should be forthcoming shortly which should be able to make some definitive statements on this interesting problem, which is interesting precisely because of its multidisciplinary applications to a wide variety and planetary – geological, biological and anthropological issues involved. Even without invoking a cometary impact, the terrain in and around the entire Nipigon basin is worth another penetrating look.

Re: number 51 – The Kiscoty-Llyodminster structure needs to be investigated immediately, but the funding for this is $0. You’re right about http://cosmictusk.com – the First Peoples accounts of this impact event may be found there and help to limit the YD transition solution space.

“Evolution of melt pond volume on the surface of the Greenland Ice Sheet” W. A. Sneed and G. S. Hamilton, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L03501, doi:10.1029/2006GL028697, 2007
“Liestїl et al. [1980] describe an emptied surface lake on Brїggerbreen, Svalbard, with a length of ~200 m and a maximum depth of ~10 m. Echelmeyer et al. [1991] report on a number of surface lakes in the upper portion of Jakobshavns Isbrae drainage basin with surface areas from a few tens of square meters up to 10 km2 and depths ranging from <1 m to 20 m in crevasses.”

In “Glacier Lakes and Outburst Floods In the Nepal Himalaya”, T. YAMABA1 & C. K. SHARMA2 Snow and Glacier Hydrology (Proceedings of the Kathmandu Symposium, November 1992). IAHSPubl. no. 218,1993. the authors describe measurements taken on Imja glacial lake. The debris covered Ice core dam at the outlet end is retaining a lake with a maximum depth of 98 meters and an average depth of 47 meters. They say ” “The ice body under the end moraine is a fossil ice of Imja Glacier,” which was deposited between the 15th and 19th centuries.

OT, but regard to the CRU and DOE funding, I amongst many spurious ones, I found a hit that appears to be credible. Summary: US DoE has been funding the CRU, and halted this pending the outcome of invetigation(s). Not settled still,

“DoE spokeswoman Stephanie Mueller said: “The Department is not currently funding them, but they do have a renewal application under review that is proceeding through the normal application process. ”

On topic, I’d give this a blue or black rating. It’s definitely not a double black; that’s reserved for when I have to dust off memories of eigenvectors and which greek letter means what mathematical concept.

So, what I’m walking away with is that there may be a repeating pattern that the YD is an example of. If you surmise that the mechanics of ice sheet disintegration as they are warmed is pretty much the same for any ice sheet, and that the follow-on effects (such as increased freshwater flow into northern oceans) will be likewise similar, given location and land-form similarities, this is entirely plausible. Any random events, such as a comet impact, or relative timings of Asian, European, or North American sheet collapse, would affect the particular shape and magnitude, but not significantly affect the relative timing and general shape.

Still thinking. If there really is a repeated pattern of gradual warming followed by a sudden dip (linked to AMOC changes), then final, inter-glacial warming, I’m thinking this lends more credence to the idea that ice sheets, probably because of their albedo, are self-stabilizing up to a point, but when some threshold is crossed, they collapse rapidly.

Lakes Superior, Huron, Michigan, and Ontario all contain regions well below sea level. These could not have been excavated by water erosion, only glacial scour. This implies that during the evolution of these lakes, an ice sheet was able to displace a great depth of water down to the bedrock. I see nothing at all to prevent an ice dam from holding back a deep lake, then eventually giving way as the climate warmed and the ice sheet shrank.

Also, the characteristics of the escape from glaciation depend on the solar insolation. T1 is similar to T3 in this regard, but their doesn’t really appear to be a YD-like event during the last deglacial event ~120 kya. Carlson (2008) attribute this to higher insolation and consequently, more rapid Northern Hemisphere ice sheet retreat and AMOC reduction. If re-routing occurred in a simlar way to the the YD then freshwater influx would have occurred in an already suppressed AMOC, resulting in little climatic impact.

I read it all the time. What, pray tell, do you think happened up in Nipigon before after the Younger Dryas? Surely some or all of that water made its way to the ground, to the oceans and to the atmosphere somewhere?

It’s the volumes, the time evolution and the periods that I am interested in. The sequences and the chronologies of the flooding events. The results are all around me here, I can’t possibly miss them. If the ‘science was settled’ then we wouldn’t be discussing it, would we. Why just a few years ago we were discussing the previous termination issues and I do remember posting a previous paper by Siddall et al. So you can hardly accuse me of not keeping up.

That being said, the ice line was approaching Nipigon before the reversal, a meltoff as rapid as that would be a sight to see of you had a spare few thousand years. It’s much better to watch it speed up to 100 years, no?

Thomas Lee Elifritz (79) — At the time of YD onset Lake Nipigon was under the Laurentide Ice Sheet. Here is an easily locatable paper:http://adsabs.harvard.edu/abs/2007AGUFMOS33A1001W
stating that the (quite interesting) Nipigon phase of proglacial Lake Agassiz evolution was quite a bit later.

This thread is about an event, YD onset, with a highly precise dating (+- 50 years) and several enigmatic aspects; several of those were addressed in the comments of the Clovis Coment thread here are RC but there is also one more which Chris Colose did not address: the noted archaeologist C. Vance Haynes Jr. has recently written a report which largely eliminates most of the purported evidence for a Clovis Comet (or other form of bolide). Too bad, because such an event would at one swoop explain the very sudden onset of YD, the extirpation of many (but not all) species of megamammels and the abrupt end of Clovis culture everywhere in North America. This doesn’t mean that there wasn’t a “meteor swarm”, but so far there is nothing definite, AFAIK. This is in conterdistinction to antlers from 43 kya blasted with micrometeorites found fairly recently in Siberia; nothing similar, of any age, found in North America AFAIK. If found and dated to within a few year of YD onset, that would be conclusive as far as I’m concerned. Of course, there is the meteor crater under the east end of Lake Ontario, “of Holocene age”, which could be more precisely dated; that would help one way or the other.

That’s why I’m interested, most of the recent work indicates that a lot of the big catastrophic glacial lake discharges occurred after the Younger Dryas, and the chronology has changed somewhat to rule out the Moorehead phase, etc. I read all those guys, Leverington, Jakobsson etc… The problem is extremely interesting even without the comet. There was massive megaflooding bursting out all over it seems, but nothing quite adds up yet because the time period is quite a bit earlier than most of the mayhem. On top of all that sea level measurements tell us something slightly different, that water had to be coming from somewhere. So maybe it was slow forcing in the beginning that just reached a resonance and then once it was forced out of that the massive flooding mixed it up enough to set it back to more or less a warming trend again. Or maybe it was something else completely. That’s why I just check in here once and a while.

I’d really like to see something definitive come out of this, and it’s getting closer all the time. I’m patient, and I found a great vacation spot out of this anyways, so I’m good.

I’m wondering what I missed in Oceanography Class about O-18. Is it that warmer SEAWATER abosorbs more of the Heavier Isotope, while warmer FRESHWATER does not?
In Fig. 4, regarding the T III Event, that would seem to be the indication – though I don’t recall being taught that, in 1982.
Go figure – that was 1982!
BTW: If you respond, could you also provide a Link for the ‘Arctic Pathway’ you mentioned? I’ve been active discusing, on several Forums, the role of the AMOC in Climate Change Scenarios, and I’d like to be able to speak as intelligently as possible about it. Haven’t seen a thing in Science, Nature, New Scientist, anywhere.

Thanks Hank Roberts! One of these days I’ll switch to Google – right now, I’m hoping Bill Gates will back a certain project of mine; so Bing it is!
Yes, yes, I know, Monsieur Moderator – nothing to do with…..

The nano-diamonds found at the boundary layers are from cometary impact, period. End of debate on whether a cometary impact occured. See in particular Napier’s paper there.

The First Peoples memories of this cometary impact event likely will provide some limit on the solution space, in particular the Mohawk account. Other limits on the YD solution space are provided by determining the areas where they survived the event, the numbers killed, and the new foods they ate and animals they hunted.

‘There was massive megaflooding bursting out all over it seems, but nothing quite adds up yet’ (81)
I love this line. It’s true that’s there’s much published on ‘megafloods’, but the reality is they’re mostly hypothetical. There’s little concrete evidence for these Agassiz floods, and the best evidence for megafloods post-dates the YD. Absence of evidence is of course not evidence of absence.
The Murton et al paper ignores the most recent ice margin reconstructions for the NW Agassiz outlet (via the Clearwater River). Their hypothesis necessitates an undocumented ice retreat/advance cycle over that height of land. This is also what would be required to permit an eastern drainage into Superior during the YD. Each hypothesis (NW or E drainage) has equal merit in my opinion. The well documented Moorhead low in the Agassiz basin establishes the fact that Agassiz was re-routed either E or NW (or both?) at some time during the YD.
Evidence that supports YD eastern drainage includes a gray clay bed in Lake Michigan that’s roughly YD in age called the Wilmette bed that looks similar to younger sediments deposited during eastern Agassiz drainage. There are gray lacustrine sediments in western Lake Superior that pre-date the Marquette Advance (10 14C BP). The gray clay has a provenance north of the Superior basin, and requires substantial ice margin retreat in the Superior basin during or before the YD. There’s also interesting/unexplained YD aged excursions in the d18O records from the Champlain Sea, Huron, and Erie that may be related to drainage.
Similar re-routing events for precursors to glacial Lake Agassiz during previous glacial terminations should have also occurred. I wonder if the Clearwater Valley isn’t in part a pre-Wisconsin landform. Ouimet Canyon near Nipigon may by another pre-Wisconsin landform carved during an early episode of glacial lake drainage to Superior.
This riddle of Agassiz routing will eventual be solved because continuous sediment records exist in the Great Lakes, and a Lake Agassiz fingerprint will be found. Until then, we’ll continue to have floods bursting all over.

Recent mapping of a number of raised beach ridges on the north coast of Greenland suggests that the ice cover in the Arctic Ocean was greatly reduced some 6000-7000 years ago. The Arctic Ocean may have been periodically ice free.

”The climate in the northern regions has never been milder since the last Ice Age than it was about 6000-7000 years ago. We still don’t know whether the Arctic Ocean was completely ice free, but there was more open water in the area north of Greenland than there is today,” says Astrid Lyså, a geologist and researcher at the Geological Survey of Norway (NGU).

In my opinion, one reason nothing has added up yet is because no one previously considered the role of impact in releasing Lake Agassiz. In my opinion, there also seems to be a research bias looking toward Atlantic data rather than Pacific data, and in particular north polar data.

The recent Canadian research on the northward drainage may bring new interest in the Alaskan and Siberian “mucks”, and their distribution and dating. We may also get some really good northern Pacific coast data as well. Afterwards, in my opinion, things will finally add up.

The real question for me is how long it is going to take for this process to unfold.